This invention relates generally to electrophotographic imaging.
In an electrophotographic imaging apparatus comprised of many components, it is desirable for the control system to have knowledge that all of the components required for imaging are present before starting an imaging operation. This is particularly true for a laser printer where some or all of these components are removable or replaceable by the user.
Laser printers have utilized an imaging system consisting of a photoconductor drum which is physically contacted by a charging mechanism (charge roll), a developing mechanism (developer roll), a transfer mechanism (transfer roll), and a scanning laser beam. In printing, the photoconductor drum is rotated within the printer. Electrical charge is applied to the surface of the photoconductor drum by the charge roll. With further rotation, the charged surface of the photoconductor drum is selectively discharged by the laser beam, depending on whether the laser is on or off as it scans over any particular point on the drum surface. With further rotation, the selectively discharged surface of the photoconductor drum reaches the developer roll, where toner is developed to the drum depending on the voltage difference. With further rotation the developed image reaches the transfer roll, where a voltage applied to the transfer roll can attract toner away from the photoconductor drum, ultimately transferring the image to the print media which passes between the photoconductor drum and the transfer roll.
The printer contains a high-voltage power supply which includes separate outputs connected to the core of the photoconductor drum, the charge roll, the developer roll, and the transfer roll. The voltage applied to the core of the photoconductor drum is controllable only as to off or on to a fixed voltage. The voltages applied to the charge roll, the developer roll, and the transfer roll are each separately controllable by the printer firmware to various voltages. There is also a feedback signal that provides a basic indication of the amount of electrical current flowing into the transfer roll, specifically whether the current is above or below a fixed pre-determined threshold. The primary reason for this signal is that the electrical resistivity of the transfer roll varies with environmental conditions, and by determining the voltage at which this current threshold is reached, the printer can get an indication of the environment and tailor operating parameters for best results.
In many laser printers, the photoconductor drum has been included in a toner cartridge supply item that also includes the developing mechanism and the toner. The printer's control system detects the presence of the toner cartridge by successfully communicating with a memory device located on the toner cartridge.
Other printers may employee a design where the photoconductor drum is not included in the toner cartridge supply item, but rather as a separate supply item known as a photoconductor unit (which may also include the charging mechanism). In many of these printers, the user cannot install a toner cartridge without first installing a photoconductor unit, so the method for detecting the toner cartridge is effective in determining that the photoconductor unit is installed in the printer.
A printer may include a separate toner cartridge and photoconductor unit, so the toner cartridge can be installed without the photoconductor unit in place. Therefore, the method of detecting the toner cartridge cannot be used to determine if the photoconductor unit is installed in the printer. Additionally, the photoconductor drum may comprise an integral part of the paper path in the printer design; attempted printing without a photoconductor unit installed will result in paper stoppage and require user intervention inside the printer to clear the paper jam. It is therefore highly desirable for the control system of the printer to have knowledge that the photoconductor unit is installed before attempting any printing operation.